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Is this a gas furnace ? Newer high eff. gas furnaces do have cooler outlet temperatures. The installer should have checked the temperature rise and gas pressure. By adjusting the gas pressure and blower speed you can obtain a higher outlet temp, but you don't want the gas pressure to be higher than the manufacture specs and you don't want the furnace to run outside of the manufactures temp rise specs.

Here's some borrowed info.:
`These are classic symptom of a limit switch opening. Can't guarantee that is what is going on, but here is some things that can cause the limit switch to open:

1) Low air flow. This can be caused by dirty filter, dirty blower wheel, dirty air conditioning evaporator coil, restricted or collapsed ductwork, blower motor running the wrong direction, blower running on too low of a speed, defective limit switch, or a combination of several of these factors. Inspection along with measuring the temperature rise will find most of them.

2) Heat exchanger failure. Split or burned through heat exchanger

3) Over-fired furnace. If you have eliminated everything in section 1 and 2, and the temperature rise through the furnace is still high, you need to check the furnace to make sure it is not burning more fuel than it is designed to burn. If it is a natural gas furnace, you can check the amount of fuel that is being burned by clocking your gas meter.

The problems you have could be potentially hazardous. If you are not able to aggressively find the problem on your own, you should hire a professional to look at the furnace immediately. The G12 furnace is getting relatively old. If you have never had it professionally inspected (sounds like you haven't), you probably should have a professional look at it immediately.' END QUOTE
This might cover your problem? don-ohio (:^)

What is your Temperature Rise across the furnace? Match this with the name plate. This tells you a lot. Is the t-rise higher or lower? If its higher, you could have restricted airflow or exhaust issues. If its lower, could be gas pressure or duct work problems, pulling in colder air somewhere. More info is needed to help. I am assuming the furnace is operating continuously when you say it will not heat over 66 degrees?

You are over heating because of dirty filter, coils, or bad or collapsed ductwork check return especially. on the units data label there should be a max and min temp rise this is the difference from incoming and outgoing air through the furnace. Check this. or it could actually be a bad switch.

Seems to me, the problem is not with gas, it's with the size of the duct work. 4" diameter is way to small if the entire duct work system is only 4". The trunk line may have not been sized proper,y. so the volume of heat has now been translated into high pressure, which is why the registers are noisy.Also 6" ducts coming off a larger trunk line to each register is the norm for 1000 sq ft home. Also, are your registers in the floor or the ceiling? Ceiling registers historically are a little noisier than floor registers. Mainly because, they're overhead and closer to your ears. But, if they were up-sized to 6", the noise would decrease quite a bit.

its the fan control AFC not shutting off , it comes on at temp rise say 150f drops back to 130f with fan on , burner shuts off fan runs untill temp falls to 100f the fan shuts off in this case it keeps running

Most likely the cause is that the furnace is firing too hot. When a furnace is installed and set up, the manifold gas pressure and fan speed has to be set by the technician so that the temperature rise through the furnace is a maximum of 50 deg F (i.e. if air enters the furnace at 68 deg F it come out no hotter than 118). The target temperature rise on any gas furnace is anywhere between 30 and 50 deg F. If the furnace runs too hot, either the manifold gas pressure must be turned down or the fan speed must be turned up to keep in this range.